The present invention relates to catalysts useful to purify air and exhaust gases containing carbon monoxide and resulting from incomplete combustion, and more particularly to a catalyst useful in removing carbon monoxide from exhaust gases from motor vehicles.
In recent years, air pollution owing to exhaust gases from motor vehicles poses a serious environmental problem, and it becomes critical to eliminate pollutants from such emissions for pollution control. Exhaust gases from internal combustion engines contain large amounts of carbon monoxide which is most harmful to the human body. Especially in cities with heavey traffic, the harmful component causes pollution with ever increasing seriousness. Complete oxidation with catalysts is found to be most effective to completely eliminate the carbon monoxide, so that various attempts have been made to develop catalysts which are useful for this purpose.
Catalysts for purifying motor vehicle emissions must fulfill the following requirements:
(1) High activity under low temperature conditions so that the catalyst is fully serviceable when the engine gives off an exhuast gas of low temperature and high carbon monoxide concentration upon starting, during idling or during low-speed or constant-speed operation.
(2) Heat resistance enabling the catalyst to remain active at high temperatures as when the engine is driven at high speeds while producing an exhaust gas whose temperature may possibly reach 800.degree. C.
(3) Outstanding mechanical strength, such as high abrasion resistance, against severe heat cycles or vibrations during operation which would deteriorate the mechanical strength of the catalyst or might release an active component from the catalyst.
(4) Resistance to poisons such as lead compounds added to gasoline as antiknock agents and sulfur compounds and halogen compounds contained in small amounts in gasoline.
(5) Activity maintained in spite of marked changes in the temperature and concentration of the exhaust gas.
(6) Stability permitting the catalyst to remain active free of thermal degradation at high temperatures.
Although researches have been carried out in an attempt to provide catalysts fulfilling such requirements, none of the known catalysts are satisfactory in these respects.
Catalysts usually employed to remove carbon monoxide are grouped into two general types: those of the platinum type and those of the metal oxide type. The former are extremely active over a wide range of temperatures and, moreover, have very high activities at low temperatures. In spite of such usefulness in their activity characteristics, however, they have the fatal drawbacks of being exceedingly low in resistance to poisons such as lead compounds, sulfur compounds and halogen compounds and subject, at high temperatures, to sintering which impairs their activity and renders them unusable for a long period of time. During use, accordingly, they must be treated repeatedly for regeneration, and further difficulties are then experienced. For example, the user is usually unable to know to what extent the catalyst has been degraded. In addition, the amount of platinum catalyst usable is limited from the viewpoint of economy and resources.
On the other hand, catalysts of the metal oxide type, although generally superior to platinum catalysts in resistance to heat and to poisons, have the disadvantages of being much inferior to the platinum catalysts in respect of the catalytic activity over a wide temperature range and exhibiting low activities at low temperatures, so that they are not practically usable. However, these metal oxide catalysts include those recently developed such as CoO, NiO, MnO.sub.2, CuO, Fe.sub.2 O.sub.3, ZnO, TiO.sub.2, Cr.sub.2 O.sub.3, V.sub.2 O.sub.5, etc. which have relatively high activities. For example, MnO.sub.2 --CuO type catalysts are very active at low temperatures and initiate oxidation of carbon monoxide at a temperature of about 65.degree. C. However, these catalysts are still low in the overall activity and are seriously poisoned with steam. In fact, they are degraded with the steam resulting from the combustion and contained in exhaust gases, so that they are not serviceable free of troubles as catalysts for purifying exhaust gases. CuO-type and CoO-type catalysts, although relatively active at low temperatures, also have a drawback. When supported by alumina which is usually used as a carrier to provide increased surface area and mechanical strength and to improve thermal stability, they exhibit a high solid phase reactivity with alumina to form aluminates, which greatly reduce their catalytic activity. Thus they are not usable with alumina which is generally the most favorable carrier.
In the preparation of catalysts using the above-mentioned metal oxides, the catalyst is shaped to a suitable size usually by sintering. To increase abrasion resistance and like mechanical strength, there is the necessity of increasing the sintering temperature, whereas when subjected to a high temperature, the catalyst can no longer retain the desired activity. Supposedly, glass frit may be employable as a binder to ensure increased mechanical strength but, even in this case, the binder bonds the particles of catalyst together, consequently reducing the catalytic activity. Thus it is impossible to improve both mechanical strength and catalytic activity; indeed the conventional sintering method involves the alternative of mechanical strength or activity, and the two requirements can not be fulfilled at the same time.